Some Humpback whales spend all winter in the Antarctic10/09/2013 15:43:32 Surprising underwater-sounds: Humpback whales also spend their winter in Antarctica
September 2013. Biologists and physicists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, found out that not all of the Southern Hemisphere Humpback whales (Megaptera novaeangliae) migrate towards the equator at the end of the Antarctic summer. Part of the population remains in Antarctic waters throughout the entire winter. This surprising discovery based on underwater recordings from the Antarctic acoustic observatory PALAOA. PALAOA is located near the research base Neumayer Station III on the ice shelf and regularly records underwater sounds of Humpback whales even in the austral winter months.
Driven by the question whether the winter-excursion of the Humpback whales in the eastern Weddell Sea was a unique event, Van Opzeeland developed a procedure for the automatic detection of Humpback whale calls and analysed all PALAOA recordings from 2008 and 2009 for acoustic signs of life from these animals. "Along with variable, high-frequency calls from the whales, our recordings also contain stereotyped calls that sound a bit like a moan. We concentrated on the latter in our analysis," the marine biologist says. "Today, we know that, in 2008, the Humpback whales were present near the observatory with the exception of the months May, September and October. In the following year, they were absent only in September. Therefore, it is highly likely that Humpback whales spent the entire winter in the eastern Weddell Sea during both years," says the scientist.
A possible explanation for the absence of Humpback whale calls during some months could be the Antarctic sea ice. "Near the observatory, open water areas in the sea-ice, also known as polynias, regularly form during winter. Such polynias form due to offshore winds which press the sea-ice off the continent out to sea. We suspect that Humpback whales use these ice-free areas. When polynias close or change position, the whales may move with them and leave the recording radius of 100 kilometres, which our underwater microphones are monitoring. However, we do not yet have proof for this behaviour," Van Opzeeland explains.
Based on the underwater sounds, the AWI scientists cannot say what the whales are actually communicating and which animals are calling in the winter months: "Possibly, the calls are produced by young whale cows that are not yet pregnant and skip the more than 7,000 kilometres long, energetically-costly migration to Africa's coastal waters. A Humpback whale-female loses up to 65 per cent of her body weight when giving birth to and suckling a calf. With this in mind, it appears energetically advantageous, from viewpoint of the young whale cows, to remain in Antarctic waters during winter. Furthermore, the coastal region of the eastern Weddell Sea likely provides krill concentrations substantial enough for the animals to find sufficient food, even in the colder season, to acquire sufficient fat-reserves for reproduction and the long trip in the following year," explains Van Opzeeland.
Van Opzeeland and her team from the AWI "Oceanic Acoustics Lab" now want to find out to which population the Humpback whales from the eastern Weddell Sea belong. The scientists are planning to compare calls from the PALAOA recordings with Humpback whale song from the coastal waters off Gabon and Mozambique. "Each Humpback whale population has its own song. Songs therefore provide an acoustic fingerprint, on the basis of which we will hopefully be able to say where the animals that spend their winters off the Antarctic continent breed," reports the marine biologist.
Furthermore, the AWI team is analysing data from a chain of underwater acoustic recorders which the Ocean Acoustics Lab scientists have moored along the Greenwich Meridian, 0 degrees longitude, between South Africa and the Antarctic continent some years ago: "We know that Humpback whales sing on the breeding grounds, as well as during their migration and that these songs alter from year to year. When and how this change happens, is however still unclear. With the help of the recordings from our chain of acoustic sensors, we may be able to shed more light on how Humpback whale song changes between years," says Van Opzeeland. She will therefore have many more Humpback whale sounds to listen to during the coming period.
The scientists report this in a current issue of scientific journal PLOS ONE.